1. Field of the Invention
The present invention relates to a sheet folding apparatus and an image forming apparatus having such a sheet folding apparatus, and more particularly, it relates to an image forming apparatus having a sheet folding apparatus in which a sheet bundle including sheets on which images were formed by the image forming apparatus is stapled and bundle-folded to form a book.
2. Related Background Art
In conventional sheet folding apparatuses, a pair of rollers are used as a folding means and sheet bundle is folded as half fold when the sheet bundle is passed through a nip between the paired rollers. When a thickness of the sheet bundle is relatively great, after the half-folded sheet bundle is passed through the nip between the paired rollers once, the pair of rollers are rotated reversely to pass the sheet bundle through the nip again to reduce the swelling of a folded line of the sheet bundle, or, the folded sheet bundle is reciprocally passed through the nip by several times to reduce the swelling of the folded line of the sheet bundle.
A speed of the sheet bundle entering into the nip between the paired rollers is set to be relatively great because the sheet bundle must be entered into the nip in opposition to pressure of the nip between the paired rollers, and it is generally designed so that the speed of the sheet bundle entering into the nip becomes the same as a speed of the sheet bundle leaving the nip.
However, in the above-mentioned conventional technique, since the speed of the sheet bundle leaving the nip between the paired roller is set to be relatively great, sufficient pressure cannot be applied to the folded line of the sheet bundle not to reduce the swelling of the folded line of the sheet bundle sufficiently. Thus, the number of reciprocations of the sheet bundle passing through the nip must be increased, thereby reducing the productivity. Further, when the sheet bundle is reciprocated through the nip between the paired rollers by plural times, the vicinity if the folded line of the sheet bundle is smudged by the sliding contact between the rollers and the sheet bundle, to thereby worsen the quality of the finished article and reducing the reliability of the apparatus.